Structural XANES characterization of Ca0.99Sm0.01TiO3 perovskite and correlation with photoluminescence emission
Graphical abstract
Highlights
► Titanium XANES measurement. ► EPR measurement. ► Photoluminescence property. ► Cluster concept.
Introduction
The physical and chemical properties of transition metal oxides have been extensively studied within the past few years [1], [2]. In particular, materials which represent the ABO3 ATiO3 (A = Pb, Ca, Sr or Ba and B = Ti or Zr) perovskite structure have constituted a central theme in the area of materials science and technology owing to their wide variety potential technological applications as fuel-cells, proton conduction [3], magnetoresistence [4], pigments [5], ferroelectricity [6], [7], [8], dielectricity [9], photocatalytic [10], photoluminescence [11], [12] and others.
As quoted by Stolen et al. [13] these structures have been termed an inorganic chameleon due to their large flexibility since the cubic mother structure easily distorts and adapts to the relative sizes of the ions forming the compound. The source of fascination is the diversity of the properties and their high sensitivity to crystal chemical tuning; i.e., a tiny change in chemical the composition or/and the crystal structure may induce huge changes in chemical and physical properties. In particular, CaTiO3 (CT) in its crystalline form displays a semiconductor behavior, and when excited by radiation above its energy band gap, which usually 3.56 eV [14] a broad greenish luminescent band appears. This material can accommodate rare-earth ions on the A-site or B-site, and this doped oxide are not only used as a probe to investigate local centers and energy [15], [16] but also to provoke changes in their optical behavior [17]. Moreover, doping foreign elements into a semiconductor with wide band gap to create a new optical absorption edge is known to be one of the primary strategies for developing materials with optical-driven properties. However, the role of the rare earth in the perovskite structure is not really clear and is still being discussed.
The spectroscopic investigations of the energy levels of samarium ions doped in different hosts have already been reported. The luminescence efficiency of trivalent rare earth ions doped into inorganic matrices depends on the energy transfer from the host to the ion [18], [19]. The Sm3+ ion has a 4f5 configuration and therefore is labeled as a Kramer ion due to its electronic states that are at least doubly degenerate for any crystal field perturbation [20], [21]. Since samarium compounds have a narrow line emission profile and a long lifetime similar to europium compounds, they can be used as a probe in multi-analytical assays [22].
The analysis of XANES (X-ray absorption near edge spectroscopy) spectra at the Ti K edge has been shown to be very sensitive to the symmetry of the titanium sitium [23]. Titanium and other transitions metals possess unoccupied electron states of d character. The analysis of the XANES spectrum may give unique information on the local and partial electronic structure and on the coordination geometry around the absorbing atom [24], [25], [26]. We found that a series of structurally disordered perovskites, synthesized by a soft chemical process called the polymeric precursor method, show intense PL at room temperature when excited by a 488 nm laser excitation line [27], [28]. We have reported several interesting properties of these ordered–disordered materials, including the fact that XANES results of the CT disordered powders [29] pointed to the coexistence of two types of environments for titanium, namely, fivefold [TiO5] square-based pyramid and sixfold coordination [TiO6] octahedron. The order was related to the presence of [TiO6] clusters, whereas the disorder was related to the presence of [TiO5] clusters. Furthermore this theory was expanded to understand the role of modifier lattice [30] and the interplay between the modifier and former lattice was refined with the concept of complex cluster [31].
In this work we apply a number of complementary experimental characterization techniques such titanium K-edge, titanium LII and LIII-edge XANES measurement and EPR measurements to obtain a clear picture of the of structural order–disorder changes in the different annealing temperature of CT:Sm samples and the relationship with the interesting optical properties of these compounds. Sm3+ ion was chosen to replace the Ca2+ ion to provoke structural defects in the compound CT:Sm through of the introduction of holes. The Sm3+ ion amount was fixed to 1% in the lattice.
Section snippets
Experimental
Amorphous and crystalline Ca0.99Sm0.01TiO3 (CT:Sm) powders were synthesized by the polymeric precursor method. In this reference [28], thermogravimetric analysis (TGA) of CT showed no weight loss for calcinations times longer than 20 h at 300 °C in an oxygen flow. These authors suggested that an inorganic amorphous material was formed. In addition, authors [28] report no weight loss and formation of this inorganic amorphous material after long calcinations time at soft heat treatment. In this
Results and discussion
Optical properties such as PL depend on both structural and electronic properties, including compositional ordering and the presence of impurities and defects. PL spectroscopy is a fundamental technique to obtain information about the electronic structure as well as optical and photoelectronic properties of the materials, including gap states and shallow and deep defects. PL depends on the electronic excitations and thus is a necessary complement to spectroscopy concerning lattice excitations,
Conclusions
In summary, the broad band visible PL emission of CT:Sm is associated with structural order–disorder. The Stokes shift decreases with increasing the thermal treatment temperature, it indicates a dependence of electron–phonon interaction on thermal treatment temperature. XANES results clearly pointed to presence of distortion in [TiO6] octahedral which could be related to two coordination modes of titanium, [TiO5] and [TiO6] structural units. EPR results support the presence of singly ionized
Acknowledgments
This work was partially supported by the Brazillian Research-Financing Institutions: FAPESP/CEPID, CNPq and CAPES. Research partially performed at LNLS – National Laboratory of Synchrotron Light, Brazil. The Ti L-edge XANES measurements were performed at the Canadian Light Source, which is supported by NSERC, NRC, CIHR and the University of Saskatchewan.
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